1. Field of the Invention
The present invention relates to a motor control device that converts AC power supplied from a three-phase AC input side into DC power and outputs the DC power to a DC link, and then converts the DC power into AC power for driving a motor and supplies the AC power to the motor, and in particular, relates to a motor control device having a life prediction unit for predicting a life of a smoothing capacitor provided in a DC link.
2. Description of the Related Art
In motor control devices that drive motors in working machines, industrial machines, forging press machines, injection molding machines, or various robots, AC power inputted from a side of an AC power source is converted into DC power by a converter, and then, the DC power is converted into AC power by an inverter, and the AC power is used as driving power of a motor.
FIG. 4 is a diagram illustrating a configuration of a general motor control device. A motor control device 100 includes a converter 101 that converts AC power from a commercial three-phase AC power source (hereinafter, referred to as just “AC power source”) 103 into DC power and an inverter 102 that converts the DC power outputted from the converter 101 into AC power having a desired frequency, which is to be supplied as driving power of a motor 104, or converts AC power regenerated from the motor 104 into DC power, and controls the speed, the torque, or the position of a rotator of the motor 104 connected to an AC side of the inverter 102. The converter 101 and the inverter 102 are connected via a DC link. In the DC link, a smoothing capacitor (DC link capacitor) 105 is provided for smoothing the DC output of the converter 101. In general, when there are multiple driving axes (feed axes and main axes), multiple motors are also provided for driving the respective driving axes. The same number of the inverters as the number of the motors are connected in parallel so as to separately supply driving power to each motor provided corresponding to each of the multiple driving axes and drive-control the motor. On the other hand, in many cases, one converter is provided with respect to the multiple inverters for the purpose of reducing cost and space of occupancy of the motor control device. Note that for simplifying the drawing, FIG. 4 depicts a single motor 104, and therefore the number of inverters 102 is one.
It is generally known that a smoothing capacitor provided in a DC link between a converter and an inverter is a part having a finite life whose electric capacity (hereinafter, referred to as just “capacity”) is reduced due to repetition of charge and discharge. When the smoothing capacitor deteriorates due to the repetition of charge and discharge and the capacity is reduced, a problem in that a ripple current flowing in the DC link increases and the fluctuation of a DC voltage is large occurs.
In addition, there are some motor control devices that reduce supply power from an AC power source to reduce the power-supply system capacity by using energy stored in a smoothing capacitor when accelerating a motor. In this case, when the smoothing capacitor deteriorates due to repetition of charge and discharge and the capacity is reduced, energy that can be stored is reduced and sufficient energy cannot be supplied during acceleration of the motor, and some type of failure may occur in a working step in the case of a working machine, for example.
Therefore, in order to replace a smoothing capacitor whose life ends at an optimal period, it is important to accurately know the capacity of the smoothing capacitor and to predict when the life will end.
For example, as described in Japanese Unexamined Patent Publication No. 8-80055, there is a method for determining a life of an electrolytic capacitor from a situation of a voltage reduction of the electrolytic capacitor provided in a DC link during power-off of an inverter.
In addition, for example, as described in Japanese Unexamined Patent Publication No. 2007-318872, there is a method for predicting a life of an electrolytic capacitor from a situation of an increase of a leakage current of the electrolytic capacitor provided in a DC link.
As described above, when a smoothing capacitor provided in a DC link between a converter and an inverter in a motor control device deteriorates due to repetition of charge and discharge and the capacity is reduced, there is a problem in that a ripple current flowing in the DC link increases and the fluctuation of a DC voltage is large. In addition, depending on the type of a motor control device, there is a motor control device that reduces supply power from an AC power source to reduce the power-supply system capacity by using energy stored in a smoothing capacitor when accelerating a motor. However, when the capacity of the smoothing capacitor is reduced, there is a problem in that energy that can be stored is reduced and sufficient energy cannot be supplied when accelerating the motor. When the capacity of the smoothing capacitor cannot be accurately predicted, the timing to replace the smoothing capacitor is missed, and the large ripple current and DC voltage fluctuation may be generated in the DC link. Alternatively, it is uneconomical because the smoothing capacitor, which has not yet reached the end of its life, may be replaced unnecessarily early. Therefore, it is important to know the capacity of the smoothing capacitor in order to accurately predict the life of the smoothing capacitor may end.
For example, according to the invention described in Japanese Unexamined Patent Publication No. 8-80055, the life of the electrolytic capacitor at a point when detecting the voltage reduction of the electrolytic capacitor provided in the DC link during power-off of the inverter can be determined, but when the life ends in the future cannot be predicted. Thus, it is impossible to make a flexible response, for example, to review an operating condition of the motor control device, on the basis of the life prediction of the electrolytic capacitor. Furthermore, it is impossible to create an efficient replacement plan of the electrolytic capacitor for the life of the electrolytic capacitor, which ends in the future. In addition, in the invention described in Japanese Unexamined Patent Publication No. 8-80055, the capacity of the electrolytic capacitor is measured during discharge, and thus, a discharge unit for this purpose is needed separately.
In addition, the invention described in Japanese Unexamined Patent Publication No. 2007-318872 is to predict the life of the electrolytic capacitor, on the basis of an experimental result in which the life ends on the third or fourth day after shifting into an increasing tendency of the leakage current. In other words, the end of the life can be predicted first just before the electrolytic capacitor completely fails, but the life that ends after a mid-and-long period (for example, in a few months or a few years) cannot be predicted. If the life that ends after a mid-and-long period can be grasped, it is possible to make a response, for example, to change an operating condition of the motor control device to one that prevents the electrolytic capacitor from deteriorating as much as possible with plenty of time until the life ends, and the deterioration prevention effect is also high. However, the life prediction in the near future, such as after three or four days, is not very effective in preventing deterioration even if the operating condition of the motor control device is changed to one that prevents the electrolytic capacitor from deteriorating as much as possible. In addition, in the invention described in Japanese Unexamined Patent Publication No. 2007-318872, the leakage current of the electrolytic capacitor is measured, and thus, a leakage current measurement unit for this purpose is needed separately.